System of electric transmission



Aug. 13, i929. H. THQMA. 1,724,437

y SYISTEN OF ELECTRIC TRANSMISSION Filed 00"'6. l?, 1925 2 Sheets-Sheet .I

l0 mb x C BA [0c [if LTI Aug 13, 1929- l P. H. THQMAs 1,724,437

SYSTEM OF' ELECTRIC TRANSMISSION Filed Oct. 17, 1925 2 Sheets-Sheet 2 WITNESSES INVENTOR @M M @Ww Patented Aug. 13, 1929.

UNITED STATES PERCY H. THOMAS, OF UPPER MONTCIIAIR, NEW JERSEY.

SYSTEM 0F ELECTRIC TRANSMISSION.

Application led October 17, 1925.

The present invention relates to systems for transmitting electric power over long distances and more particularly with regard to those systems in which it is desired to transmit very large amounts of power. It has been the universal custom in such cases to make use of the well known three phase system using three line conductors, as the simplest and best system for such transmission. With the development of the electric industry, however, and particularly where power must be transmitted for long distances, it is found that a greater power capacity in a single circuit than can be secured by the three phase system is to be desired and the present invention has for its purpose to provide such increased capacity.

In one principal embodiment of my invention I make use of four transmission conductors in a` single circuit with four phase currents. While I do not claim to be the first to use a four phase circuit for power transmission, it will be found that my invention shows new combinations and improvements in such systems having very material' advantages over the prior art as will appear hereinafter.

IVhere important blocks of power are transmitted long distances and where continuity of power is of greatest moment, it .is necessary to provide as well as possible against accidents, such as lightning, insulator breakdowns, etc., and to so lay out the system, that when such accidents occur the least possible inconvenience will be caused. Furthermore, when such lines are operated at loads approaching their theoretical maximum capacity, for example, by supplying wattless kva. at the receiving end to produce a favorable power factor` they become very sensitive to voltage and load disturbances so that precautions must be taken. My systems are favorable from this point of view.

In some of my systems I transform energy `generated as six or four phase energy to four or three phase energy for transmission, and afterward transform it again for utilization. Other features of my invention will appear hereinafter.

In the drawings, Fig. l represents a simple embodiment of my system and Fig. 2 a modification. Fig. 3 represents a system adapted to transmitting very large amounts of power in a staple and economical manner and Fig. 4 shows a Special use of the system of Fig.

Serial No. 63,078.

3 and Fig. 5 is a modification. Figs. 6. 7 and 8 are illustrative diagrams. l

In Fig. l, a four phase generator is shown at 11, feeding four single phase transformers, supplying them potentials ofv phases 90 deg. apart. la, 2a, 3a and 4aV are the primary windings and 1b, 2b, 3* and 4* are the secondary windings. I prefer to connect together and ground the middle pointsl of the secondary windings at 26. 10a, 10b, 10c and l0d are four transmission line conductors which are assumed to be mounted on suitable towers. Single pole circuit breakers 45, with controlling relays which may act independently of one another, are located at each end of each line conductor. Thus if desired, one line conductor may be cut out without interrupting the others. At the receiving end of the transmission, these line conductors supply four step down transformers as shown, threse transformers being of equail kva. capac1ty, 5, 6a, 7 n and 8a being the primary windings and 5", 6b, 7b andSb being one set of secondary windings and 5c, 6, 7c and 8 being another set. As in the case of the enerating station I prefer to ground the micIdle points of the high tension windings at the receiving end as at 25a. The connections for the secondaryl windings are as shown, the secondaries 7b, 7C and 8", 8c each consisting of two equal voltage coils, one coil of each winding being of half the current carrying capacity of the other coil of that winding.

As the great majority of existing distribution circuits fed from transmission lines are three phase, I arrange the transformer secondary windings 5b, 6h, 7b and 8b to supply such circuits as shown at A, B, and C. The secondary windings 5* and 6b are wound to deliver a voltage equal to 87% of the voltage delivered by the winding 7 b; 5b and 6b deliver, voltages of the same phase.

The windings of 8b are of the same phase as those of 7b. IVith the connections as shown for these transformers and as illustrated in Fig. 7, the circuit naturally gives six phase power and when thus used provides a complete and efficient use of the transformer capacity and this without disturbance of the natural power factor by the method of connection as occurs in some connections for phase transformations. However, as shown in full lines in Fig. 7, it is not well suited for supplying three phase distribution circuits, for all Six phases must supply power to fully utilize the transformer capacity. I, therefore, add two single phase auto-transformers 40 and 41 across certain of the six leads, shown dotted in Fig. 7, con- .,nected as shown in Fig. l, the middle points of which will be at neutral potential. By

connecting these middle points to the neutral bus 39 which is also connected to the connecting` point for two coils of the windings 7b and 8b. three phase power can be taken off `at A. B and C or', if desired, from the remaining three of the siX busses' shown. t

l show at 9 a synchronous type machine which may be a motor or a generator or synchronous-condenser, supplied by six phase power from the second set of secondary i ao windings 6, 7c and Se, which are connected similarly tothe windings 5b, 6b, 7b and 8b, certain of which connections desirable for three phase service being omitted. It will be understood that all three sets of windings,

s primaries and secondaries with the same refload on .A, B, C over the four linev wires 10a, 10b,.l0c and, 10d.

erence numeral arson' the same transformer l cores and are supplied respectively from the Vthe two serially connected coils of the transformers 7b and 8b is 87% of that on the trans-v former 5b. This is the proper proportion of the i voltage of the two transformers with cores excited at phases 90 deg. apart to give vthree phase voltages, but with somewhat reduced transformer capacity. This relation may be seen in Fig.' 7. While with the lswitches 44 open only half the transformer capacity is in use for load on three of the six leads, the load will still be sufficiently well balanced if the six phase machine 9 is propverly designed, for this machine will go far as is well known toward equalizing the load between the Aphases andf on the four line wires. It may be pointed out that in addiion to thetendency for the ksynchronous machine 9 to balance the load on the four conductors and hence in the transformer secondaries 5b, 6b, 7l and 8b, the interconnections between the latter coilsare such as inherent fly to tend to distribute three phase energy over all the line conductors; for example the `on the loadv phase ABV is supplied naturally from the secondary 5b, but as will be seen by J a reference to F ig.7 there is a parallel path for the supply of current, viz the path 7 b, 6b, 8l?, which is in parallel to 5b and of the Same voltage This parallel path has a higher v impedance but will take va material sjharerof` the load current, the actualY energy being tween conductors are Current taken supplied by the secondary 6b, since the current and the voltage in 7u and 8b are out of phase. Current in load phase AC is supplied partly by the secondary 5b and partly by the secondaries 7b and 8b, see Fig. 7. The current from A flows in parallel paths lirst by secondary 5b `and central secondary 8b and second by secondary 8b on the left and the secondary 6b. Similarly with load phase BC. Thusthree phase load current in ABC causes a iiow of energy over all four of the line conductors but in unequal quantities so that there will be a reduction of capacity below the combined individual transformer capacities. The balancing` effect of the. synchronous type machine will tend to eliminate Vthis imbalance as stated above.

The connection between the middle point of the winding 6b and the middle point of the winding 8b is similar to that just described for 5b and permits three phase current to be taken from the six unlettered busses of Fig. l. y i

Fig. 7 shows the vector relationV of the voltages in the windings 5C, 6c, 7 and l8c of Fig. l, the iigures onthe right hand side indicating voltages in percent of the voltage to neutral and the figures on the left indicating the fcurrent in per centV of the current in the line conductors. The ligure is symmetrical;

The machine 9 may be relied upon for voltage regulation of the receiving end of the line either alone or in conjunction with other machines.

rEllis system of Fig. l has many advantages. Assumingthat each line has the same voltage to ground, the four conduftors will transmit 1/3 more energy than the usual three phase system. lvhile the maximum voltage. Vbetween line conductors will be raised `by about 15%, this is of no material importance since the clearance distances bewliolly by mechanical considerations and bccause in the actual transformers and oil breakers, insulation has to be provided primarily for the voltage to ground and' not for that 'between line conductors.

The use of synchronous machines connected to the secondaries on the main line transformers at both ends of the line is cf real value in increasing the stability of the lt is well linownrthat when a sudden load comes upon a long line which is very heavily loaded and which is laid out to have its receiving end voltage-maintained by an automatic control of the out of phase lrva. supplied to the line from a suitable source vat the receiving end, a demand for an inquires an adequate capacity to supply1 such emergency kva. quickly and any impedance determined almost to the free flow between machine and line is harmful. The special line transformer windings and the phase relations chosen are particularly favorable for stability. For example, the synchronous condenser 9 of Fig. 1, which has the capacity to supply ont of phase liva. automatically to the line in case of extra heavy load, or of some accident throwing a heavy lagging ltva. on the line, is connected to the line directly through the transformer coils 5d. 5C, 6a, 6c, etc., so that a. minimum of impedance intervenes. Since eaih transformer has its own core a heavy flow of current in one phase of the four either in the transformer or in the machine 9 has little effect on the other three. Furthermore, a heavy flow of lagging kva. on the load circuit will cause a great drop in the supplying transformer coils and. were the synchronous machine 9 connected to this load circuit directly, as is often the actual case instead of a separate set of transformer sccondaries as shown, this lagging kva. would greatly weaken the field strength of the. machine S), and lessen its power to supply hva. to the line. The secondary coils 5b, Gb, etc.. thus serve a cushion to protect kthe stability of the line.

The fonr wire system here used permits the use of two transformers'instead of four where preferred` thus saving costs, since. the windings 1" and 2h may be vgiven the same phase by reversing connections and hence may be put on the same core. similarly with the windings 3b and Lll. That is, the two transformers 1?-1), and 2a-2, for example may be combined into one transformer, all coils being wound on the same core but since the potentials of the two corresponding line conductors 10 and 10`D must be opposite, while thel voltage induced in the coils 1.b and 2b supplying these conductors by the magnetism of the core is the same in both, they must be oppositcly connected, that is the start of one winding will be connected to ground while the finish of the other is so connected. Similarly the four coils 3, 3, la and 4b may be combined in a second transformer.

In Fig. 2 I show an alternative method of arranging the stepdown transformers of Fig. v1, giving a performance of the synchronons machine 9@ more favorable from some points of view.

Considering Fig. 2, the primary windings of the stepdown transformers 5a, 7 and 8 are the same as in Fig. 1. The synchronous machine 9n is four phase and is snpplied `by the stepdown transformer secondaries 5, 6d, 7d and 8d and with such connections and at such voltages that each winding feeds a certain 25% section of the armature, for example, winding 5 feeds section 5f, 6d feeds (if, etc.; the net result being normally a complete and symmetrical loading of the machine. Should, however, trouble occur on the circuit of one of these armature sections its effect on the machine as a whole will. be rela ivelv small and much less than were each Vwinding` wound for the full machine i-*oltage and connected across a group of armature scc-tions.

If desired a four phase local power circuit or two single phase circuits may be fed from the buss D. E, F and GA.

The secondary windings 50. 6, 7 and 8e corresponding to the primaries 5, 6, 7a and 8, are connected to give three phase cnrrents at H, I and J for general distribution. For this purpose the voltage on the windings 5e and 6e which are connected in parallel are 100% and the windings '.70 and Se are wound for 87% so that with the connection shown, symmetrical three phase voltages will be produced at H. I and J. This connection has already been described above except that transformers are here used in pairs.

It will be noticed in Fig. 2 that each line conductor, as 10a, 10, 1()c or 10d has its own transformer both in the receiving and sending end and that the secondary windings 5, 6d. 7d and 8d are all connected to separate sections of the windings of the machine 9u. Similarly with the primary windings 1a, 2, 3"J and fla, and the machine 11. vOne advantage of this is that a ground on one line conductor or trouble in one transformer or similar trouble will be largely confined to one section or leg of the system and will have arelatively small effect on the balance of the system, which is of considerable importance where lines are operated at or near their maximum capacity. For example. a bad ground on the conductor l01 near the receiving' end will drop the voiJ ge on the corresponding step-down transformer affecting the voltage in the secondary winding 5d, but to a less extent due to 'the transformer impedance, and reacting on the scction 5f of the winding of the machine 9, but not greatly affecting the voltages in the rest of the system.. The field oemagneti/.ing action of the current in 5 will of course, affect the field strength some-what and thus the general machine voltage.

In Fig. 3 I show a system particularly well adapted to very large. capacities. 18 is a six phase generator feeding the six primary windings of six transformers, 12, 153, 14, 15a, 162L and 17, each winding being fed by a small section of the arma-ture winding of the generator 18, but the whole system serving to produce a. symmetrical loading of the machine. The phases of the transformer primary winding potentials, talien in the order of the reference numerals, are consecutively in order and spaced apart (30 deg. The secondaries of these ti formt-r are 12", 13b, 14h, 15b, 16b and 17h, arranged so that their normal positive maxima takenages and phases can `be determined from in order of the reference numerals and in the directions of the arrows will occur' at 60 deg. intervals. The connections between the windings are shown, these transformers receiving six phase and delivering four phase currents, the point m and g/ being located .577 of the winding of the windings 13 and 16 from the neutral point of the high tension system 19 to the coil terminals. The four line conductors are 10, 10, 10 and 10. A single pole circuit breaker is located at each end of each line. The automatic relays for these breakers may operate independently on each conductor, so that one conductor in trouble may be cut out without disconnecting the others. The transformer windings 12, 14, 15 and 17 are wound for a voltage of 115% of the windings 13 and 16. The four line wires will have four voltages to Iground their phases succeeding in order by intervals of 90 deg. as in case of the line conductors of Fig. 1.

rlhese transformer secondaries will usually be wound for very high voltages. There are six stepdown t-ransformers at the receiving end with primary windings 27 28, 29, 30, 31 and 32. These primary windings are wound and connected in the same manner as the secondaries of the step-up transformers 12, 13, 14, 15, 16 and 17.

The secondary windings 27, 28, 30, 31, 29, 32 are connected at their middle points by the conductor 42 and taken in pairs deliver siX phase power to supplythe six phase synchronous machine 24 as shown. second set of secondary windings 27, 30, 28, 31, 29 and 32 are provided similar to the first set to supply power to a three phase general distribution system N, P, Q, somewhat as in the case of Figs. 1 and 2. The circuit of the synchronous machine 24 should be adopted to voltages suitable for machine operation and local distribution, while the circuit N, P, Q, is suitable for a wide spread distribution system, for example, distribution at 110 kv. in cases where 220 kv. is utilized for transmission on the conductors 10 to 10. In this system of Fig. 3 the plan of electrical separation between the legs of the transmission circuit is utilized but not perfectly, since the legs are somewhatinterconnected by the transformers.

Fig. 6 shows, in connection with Fig. 3, the relations of the phases and voltages and currents in the transformers windings 12, to 17 and 27 to 32. In Fig. 6, figures on the right hand side are voltages in percentage of line leg voltage to neutral and the figures on the left hand side are currents in percentage of line current. The figure is symmetrical about both horizontal andV vertical axes sol that all the currents, volt- Y the diagram.

One important function of the synchronous machine is to supply supporting kva. to the line at times of overload orl disturbance as well as to supply normal charging current to the line and to correct power factor and this function is well performed either by the four phase or by the six phase machine with the direct and effective transformer connections, and especially where the legs of the circuit are kept separate since each phase of the machine is relatively free to exert its own supporting action.

The system of Fig. 3 has further great advantages. For example, should one line conductor fail and this conductor be disconnected, as for example 10, there would still remain a complete polyphase system suitable for maintaining synchronism, which would transmit the full original load except in cases where the four conductor system was operating near its theoretical maximum capacity before the loss ofthe line conductor 10. That is, if the system of Fig. 3 be assumed to have a maximum theoretical capacity of 6,000 kw. for example, and be assumed to be operating initially at a load of 4,000 kw. and if it be assumed that line conductor 10 be disconvoltage impressed on the other three line conductors which will continue to pass current to the receiving transformers which will continue to be excited and to receive energy over the three good conductors. The amount of energy transmitted by the good conductors will, however, increase until the aggregate is equal to the original 4,000 kw. and no transformer will be seriously overloaded. Furthermore, the load currents-Will be polyphasel though not symmetrical. With the usual three phase system, except for ground currents, a broken conductor results in single phase transmission.

As in theV case of Fig 2, trouble on one line wire will cause less disturbance in the system than if the windings, 12 to 17 and 27 to 32 were connected to an inter-related group of armature sections inthe synchronous machines 18 and 24 such for example as the well known closed loop type of generator winding. For this latter case the windings would usually be wound for a higher voltage.

In Fig. 4, I show an emergency use of the system of Fig. 3, such for example, as might occur when one transmission conductor as 10,is disabled, leaving three good conductors for service. In Fig. 4, the same connections are used as in Fig. 3 for the generator 18 and primary windings 12, 13, 14, 15, 16 and 17, but the secondary windings are connected in parallel in pairs, those havnected, there will be no great change in the ing the same phase beingparalleled, viz, 12b and 15b, 13b and 16D, 14h, and 17b as shown. One end of each of thesedouble windings is connected to a. neutral conductor 25 which may be grounded as at 26 and the other ends to the line conductors 10, 10b and 10c as shown, this making a normal three phase transformation, At the receiving end the transformer primary windings 27, 30, 28, 31a, 29a, 32'd of Fig. 3 are reconnected in the same manne-r as shown for the similar secondary windings 12b, 15b, 13b, 1Gb, 141 and 17b at the generator endand separate illustration is not necessary. The remaining portions of Fig. 3 and Fig. 4 are the same except for the use of taps in the low tension windings of certain transformers described below, this including the transformer secondary windings 27" to 32D-, and 27c to 32c of Fig. 3.

It will be noted that, if with the connections of Fig. 4, the normal voltage be maintained on the generator, voltage between line conductors will be increased in the ratio of approximately over the line voltage of Fig. 3 and the power that can be transmitted will be increased in the square of this ratio or so that the same power may be transmitted as with the four conductors of Fig. 3 though at a 15% increase in voltage. For this result to be secured it is assumed that the low tension windings 13a, 16a, 28a, 31a and 28c and 31c have in use a portion of the winding equal to a part of the whole, so that the high tension voltage may be the same in all phases. If this increased potential is objectionable, the voltage at each end is under control through the field currents of the synchronous machines, such as 18 and 24, so that lower voltages may be used. If lower voltages on the line are to be used without disturbing voltages on the low tension side of the transformers, taps or ratio changing devices may be used. f

The cores of the transformer windings 13b and 16b in Fig. 4 will be excited to 115% of the `normal with normal generator voltage, but their average current will be reduced as compared with Fig. 3.

By providing somewhatmore than the normal proportion ofcopper for the current appropriate to the normal operation of Fig. 3 in that portion of the windings 13b and 16b lying between the taps and ground, the

total losses in these transformers can be kept near normal during the emergency operation of Fig. 4. The voltage and losses in transformer windings 12b, 14h, 15b and 17b will be normal.

At times of light load or by permitting temporary overloading of part of the equipment, the change from the connections of Fig. 3 to that of Fig. 4 may be accomplished without interrupting service. For example, after disconnecting conductor 10", leaving the other three conductors to carry the load, the transformer secondary 15b may be reconnected between the neutral point- 19, see Fig. 8 and the conductor 10". The secondary 12b may then be connected in parallel with 15vb between the neutral point 19 and the conductor 10". The conductor 1()c may then be freed and the winding 13h paralleled with the winding 16h. The winding 14b may then be connected between the neutral point 19 and conductor 10c and the conductor 10d disconnected. The winding 17b may then be connected in parallel with the secondary 14h, which will give the system of connections shown in Fig. 4.

Fig. 8 shows phase relations of the several windings in Figs. 3 and 4, during these changes, the normal current and voltage relation for Fig. 3 being shown in Fig. 6. The dot and dash lines in Fig. 8 show the new connections for Fig. 4.

In Fig. 5 I have shown an alternative arrangement to Fig. 3 in which I have three step-down transformers in place of six; two of these having pairs of similar primary windings 35a and 35b and 37a, 37b and the third one primary winding 36ay The former windings have a voltage 1.15 times the voltage of one half of the winding 36a. These windings are connected as shown to receive four phase power from the conductors 10a, 10b, 10c and 10d, which are the line conductors of Fig. 3 and magnetize the core of the three transformers in three phase relation. The taps in the transformer primary 36a are .57 7% of the winding from the middle point which is grounded, to the terminal.

Three secondary windings 35C, 36e, 37 C are connected to produce six phase currents, by connecting the middle points together. These six phase currents are connected to a six phase synchronous machine (not shown) as in the case of Fig. 3.

A second set of secondary windings 35, 36d and 37d are connected in star as shown to supply three phase distribution circuits.

In general it may be said that the three phase windings provided for general distribution cause a certain degree of interconnection between the six phase secondaries on the same cores but this is relatively small on account of the reactance between coils in a transformer.

; which the length of line and its maximum vpower capacity and limit ofsynchronizing power as well as the control of power factor taking account of the charging current are the dominating factors in the system and such a line I may call along line. In other cases different characteristics may be dominating and different design principles may apply. n 1- While I have illustrated my invention .t with a four conductor transmission line, the

principles of the invention may be used and applied in lines having more than four conductors as will be evident to those skilled in the art and I wish to include any such increased nuinber of phases undermy claims as far as they are applicable.

I claim as my inventionl. In a system of electrical distribution means for equalizing three phase energy in a polyphase system, comprising two transformer cores inagnetized at 90 degrees and carrying one set of windings co-operating with a source and a second set of windings for said three phase energy connected as follows: on the first core a winding connected between the lst and 4th terminals of a series of .6 terminals for six phase voltages, also Vtwo windings one half the voltage of said-*winding connected respectively be tween the 2nd and 3rd, and the 6th and 5th terminals; on the 2ndv core two windings of 86.6% ofthe voltage of the lst winding connected respectively between the 2nd and 6th, and v8rd and 5th terminals and connections at the middle of said two last named windings'to the 25% and 75% points respectively of the winding connecting terminal 1 to terminal 4, together with two auto-transformer windings on separate cores connected dividual transformer for each phase of said transmission line, two sets of secondary windings for said transformers one set supplying six phase current to a synchronous type machine and a second set similarly connected for supplying six phase current, each of said sets of secondaries including twoA similar coils developing voltages of the same phase as one phase of the voltage of the transmission line, two pairs of similar coils, one pair having twice the current capacity of the other pair' all ldeveloping voltages of a phase at 90O from that of the first two coils, the voltages developed by the rst named coils being twice 87% of the voltage of each of the other coils, connections for supplying symmetrical six phase voltages from these coils and means for distributing energy from a three phase load substantially uniformly over said line and said transformer windings, comprising autotransformers connected across two diagonals of said six phase voltages and having their middle points connected to the neutral point of said six phase secondaries. Y

8. Transforming means for changing four phase power to six phase power, comprising four single phase transformers constituted as follows-two transformers having similar windings with voltages of the same phase and two other transformers having together two pair of windings the two coils of a pair being similar and all having voltages of a phase 90o from said first named voltages and all having voltages bearing ratios to the voltages of said first named coils as .5 is to .87, the current carrying capacity of thecoils of one pair being twice that of the coils of the other pair, the last named transformers having each one coil of each pair, together with connections joining the middle points of the first named coils and the coils of the pair having the larger carrying capacity and between the ends of the first named coils and the coils of the other pair; also a connection between the two coilsof the pair having the larger carrying capacity.

4. A polyphase transformer connection comprising a plurality of windings as follows two windings of the saine phase having a voltage of /in relative magnitude;

' two windings of a phase90 degrees from the phase of the-above windings and having a voltageof .5 in relativfJ magnitude, the middle point of each winding being Vconnected to the middle point of one of the .5 windings, two Vadditional windings similar in voltage to the .5 windings of the same phase, these last named windings being con p nected respectively between the similar ends of theN/ windngs and a. connection between two ends of the first named .5 windings, putting them in series, together with two additional windings connected from one end of one of theN/.coils to the other end of the other coil and at its mid-point to the PERCY H. THOMAS. 

